The present invention relates generally to electrical capacitors and particularly to a crater-style capacitor for high-voltage radio-frequency applications.
Many applications require the sampling and control of high-voltage waveforms, particularly waveforms at radio frequencies. A sampling capacitor may be used to sample the waveform as feedback to control the amplitude of the waveform. To provide accurate sampling and amplitude control, the sampling capacitor must have a stable capacitance value. Further, it is often desirable to provide a guard capacitor positioned concentrically outward of the sampling capacitor. The guard capacitor protects the sampling capacitor from the effects of fringing fields and allows for accurate amplitude control based on the capacitance of the sampling capacitor only.
Desirable attributes of a sampling capacitor include negligible self-heating between the capacitor electrode and dielectric and elimination of corona discharge at the electrode edges. Self-heating of the capacitor can result in damage to the capacitor dielectric, and as the capacitor temperature rises it becomes more difficult to compensate for changes in capacitance due to the heating of the dielectric. Corona discharge at the edges of the electrodes can destroy the capacitor, damage the dielectric, or start a fire.
The crater-style capacitor of the present invention eliminates corona discharge at the electrode edges and has negligible self-heating. A system incorporating the capacitor can easily be compensated for stability. The capacitor includes two concentric capacitors patterned onto a dielectric such as quartz or ceramic. The outer guard capacitor protects an inner sampling capacitor from fringing fields. Amplitude feedback to a control system incorporating the capacitor is provided by the sampling capacitor only. In some applications, the combined capacitance of the guard capacitance and the sampling capacitance can be used in a resonant RF tank circuit.
Previous designs for sampling capacitors include ceramic-gap capacitors, open-gap capacitors, capacitors having a planar upper electrode, capacitors with an encapsulated upper planar electrode, and capacitors with a half-dome input connector to cover the upper planar electrode in combination with encapsulation of the upper planar electrode.
The disadvantage of ceramic-gap capacitors is that having ceramic in the gap yields a large spread of temperature coefficients and capacitance values. Ceramic is a variable blend with properties that vary from batch to batch of ceramic material.
Open gap capacitors are very large and suffer from voltage range limitations caused by corona discharge at planar electrode edges. Mechanical spacers used in the open-gap capacitor have varying temperature coefficients that alter the capacitance values. The dielectric constant of an open-gap capacitor is affected by humidity unless the capacitor is placed in a vacuum or other controlled environment.
In FIG. 1, there is provided for purposes of illustration, a prior art sampling capacitor generally designated as 100. The sampling capacitor 100 has a dielectric 103 with an upper planar electrode 115 and a lower planar electrode 119 positioned on opposing surfaces of the dielectric 103. Additionally, guard electrode 121, completely encircles the lower planar electrode 119. A sampling capacitor is defined by the upper planar electrode 115 and the lower planar electrode 119 and a guard capacitor is defined by upper planar electrode 115 and the guard electrode 121. An electrode gap 135 electrically isolates the guard electrode 121 from the lower planar electrode 119. One disadvantage of sampling capacitor 100 is that corona discharge occurs at an upper edge acuity 116 of the upper planar electrode 115 at operating voltages above about 2.5 kilovolts. In one experiment, encapsulating the upper planar electrode 115 with an encapsulant 125 extends the operating voltage to about 8.0 kilovolts; however, self-heating occurs at about 8.0 kilovolts. Further, as illustrated in FIG. 3, at operating voltages near about 12.0 kilovolts, an intense voltage gradient 118 at the upper edge acuity 116 create a corona discharge (hot spot) that ignited the encapsulant 125.
FIG. 2 illustrates another prior art sampling capacitor generally designated as 200. The sampling capacitor 200 employs a half-dome button 224 to overcome the intense voltage gradient at an upper edge acuity 216 of an upper planar electrode 215. The half-dome button 224 eliminates the self-heating and corona discharge problems of the sampling capacitor 100; however, penetration of encapsulant 225 between the half-dome button 224, as illustrated by arrow 227, results in a button-to-ground capacitance through both the encapsulant 225 and dielectric 203. Because the button-to-lower electrode capacitances do not manifest themselves totally through just the dielectric 203, the composition and temperature coefficient of the sampling capacitor varies. Therefore, the sampling capacitor 200 yields a high-voltage capacitor, but with the value of the sampling capacitor compromised by the button-to-ground capacitance.
From the foregoing it will be apparent that there is a need for a sampling capacitor that can withstand high-voltages, produces negligible self-heating, eliminates corona discharge, reduces the encapsulant's contribution to the sampling capacitor, and allows for a system incorporating the sampling capacitor to compensate for system stability.